In recent years, the demand for diesel oil is increasing gradually in the world. However, the limited diesel oil resource is decreasing gradually, and therefore, a trend of insufficient diesel oil supply and increasing price is occurred. Additionally, because the molecular weight of paraffin in the components of diesel oil is relatively large, the combustion ratio is not high enough, and the combustion performance is not good sufficiently, not only the diesel oil consumption is increased, but also the pollution degree of exhaust to the environment is aggravated. In order to attain an object of energy saving and environmental protection, the research on diesel oil additives in the technological and industrial fields all around the world is enhanced. Polyoxymethylene dimethyl ethers (RO(CH2O)nR) has very high cetane value and oxygen content (42%-49% for the methyl series and 30%-43% for the ethyl series). The addition of polyoxymethylene dimethyl ethers into diesel oil in an amount of 10%-20% can improve the combustion characteristic of diesel oil prominently, increase the heat efficiency effectively and reduce the discharge of NOx and microparticles. In view of the vapor pressure, the boiling point and the solubility in oil products thereof, polyoxymethylene dimethyl ethers (DMM3-8) with 3≦n≦8 is generally used as an oil product additive Polyoxymethylene dimethyl ethers (DMMn) was early synthesized by taking methanol, formaldehyde, polyformaldehyde or glycol ethylidene formal as raw materials under a catalysis of sulfuric acid or hydrochloric acid. In recent years, BASF (WO 2006/045506 A1, CA 2582502) obtained DMM3-8 with a content below 26% by reacting at 100° C. for 8-12 h in the case of taking a protonic acid, for example, sulfuric acid, trifluoromethylsulfonic acid or the like, as catalyst and taking methylal and trioxymethylene (or polyformaldehyde) as raw materials. The method has a rigorous reaction condition, a low conversion ratio and a low content of the component which can be used as an oil product additive. In 2005, De Gregori etc. (EP 1505049 A1) synthesized DMM3-8 with a reaction under N2 of 1.0 MPa using a protonic acid catalyst. The reaction time was reduced greatly and the DMM3-8 yield could be up to 51.2%. The above catalysts have the disadvantages of serious corrosion, difficult separation incapable circulating application, large processing energy consumption, unreasonable product distribution, low trioxymethylene conversion ratio, and low selectivity for the component of DMM3-8 which can be used as an oil product additive, and the like. BP (U.S. Pat. No. 6,160,174, U.S. Pat. No. 6,265,528 B1) performed a research on multiphase catalysis and synthesized DMM3-8 with a content of only 11.6% in the case of using methanol, formaldehyde, dimethyl ether and methylal as raw materials. Because the method has a low catalyst activity and a complex process, it is difficult to be industrialized. Being a new green solvent or catalyst, ionic liquid has become a hotpoint of the green chemistry research due to the unique advantages of adjustable polarity, wide liquid range, high thermal stability and almost neglectable vapor pressure or the like. Ionic liquid has been widely used in some important aldolisations. In 2007, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences reported a method for synthesizing DMMn by a reaction of methanol and trioxymethylene taking an ionic liquid as catalyst, wherein the reaction conversion could be up to 90.3% and the selectivity for DMM3-8 could be up to 42.6% (CN 200710018474.9).